In "Heterocyclic Compounds, The Pyrimidines, Supplement 1" (1970) Wiley-Interscience, page 119, it is stated that "Chlorination using elemental chlorine is seldom easy in practice and yields are rather indifferent." However, pyrimidines having amino, hydroxy or methoxy activating groups have been chlorinated under carefully controlled conditions, e.g. 2,4-diamino-6-chloropyrimidine and 4,6-dihydroxy-2-phenylpyrimidine. In "Heterocyclic Compounds, The Pyrimidines" (1962) Wiley-Interscience, page 8, it is taught that the introduction of election-releasing substituents into any position of the pyrimidine ring makes the 5-position readily halogenatable.